Chlorine (IPA: [ˈklɔːriːn], Greek: χλωρóς chloros, meaning "pale green"), is the chemical element with atomic number 17 and symbol Cl. It is a halogen, found in the periodic table in group 7. As the chloride ion, which is part of common salt and other compounds, it is abundant in nature and necessary to most forms of life, including humans. In its elemental form under standard conditions, it is a pale green gas about 2.5 times as dense as air. It has a disagreeable, suffocating odor that is detectable in concentrations as low as 3.5 ppm[1] and is poisonous. Chlorine is a powerful oxidant and is used in bleaching and disinfectants. As a common disinfectant, it is used in swimming pools to keep them clean. In the upper atmosphere, chlorine atoms have been implicated in destruction of the ozone layer.

Chlorine gas is diatomic, with the formula Cl2. It combines readily with nearly all other elements, although it is not as extremely reactive as fluorine. At 10 °C one litre of water dissolves 3.10 litres of gaseous chlorine and at 30 °C only 1.77 litres.[2]

This element is a member of the salt-forming halogen series and is extracted from chlorides through oxidation often by electrolysis. As the chloride ion, Cl−, it is also the most abundant dissolved ion in ocean water.

Chlorine gas, also known as bertholite, was first used as a weapon in World War I by Germany on April 22, 1915 in the Second Battle of Ypres. As described by the soldiers it had a distinctive smell of a mixture between pepper and pineapple. It also tasted metallic and stung the back of the throat and chest. It was pioneered by a German scientist later to be a Nobel laureate, Fritz Haber. It is alleged that his role in the use of chlorine as a deadly weapon drove his wife, Clara Immerwahr, to suicide. After its first use, it was utilized by both sides as a chemical weapon.

In nature, chlorine is found mainly as the chloride ion, a component of the salt that is deposited in the earth or dissolved in the oceans — about 1.9% of the mass of seawater is chloride ions. Even higher concentrations of chloride are found in the Dead Sea and in underground brine deposits. Most chloride salts are soluble in water, thus, chloride-containing minerals are usually only found in abundance in dry climates or deep underground. Common chloride minerals include halite (sodium chloride), sylvite (potassium chloride), and carnallite (potassium magnesium chloride hexahydrate).

Chlorine has isotopes with mass numbers ranging from 32 g mol−1 to 40 g mol−1. There are two principal stable isotopes, 35Cl (75.77%) and 37Cl (24.23%), found in the relative proportions of 3:1 respectively, giving chlorine atoms in bulk an apparent atomic weight of 35.5.

Trace amounts of radioactive [[Chlorine-3636Cl]] exist in the environment, in a ratio of about 7x10−13 to 1 with stable isotopes. 36Cl is produced in the atmosphere by spallation of 36Ar by interactions with cosmic rayprotons. In the subsurface environment, 36Cl is generated primarily as a result of neutron capture by 35Cl or muon capture by 40Ca. 36Cl decays to 36S and to 36Ar, with a combined half-life of 308,000 years. The half-life of this hydrophilic nonreactive isotope makes it suitable for geologic dating in the range of 60,000 to 1 million years. Additionally, large amounts of 36Cl were produced by irradiation of seawater during atmospheric detonations of nuclear weapons between 1952 and 1958. The residence time of 36Cl in the atmosphere is about 1 week. Thus, as an event marker of 1950s water in soil and ground water, 36Cl is also useful for dating waters less than 50 years before the present. 36Cl has seen use in other areas of the geological sciences, including dating ice and sediments.

Chlorine can be manufactured by electrolysis of a sodium chloride solution (brine). The production of chlorine results in the co-products caustic soda (sodium hydroxide, NaOH) and hydrogen gas (H2). These two products, as well as chlorine are highly reactive. There are three industrial methods for the extraction of chlorine by electrolysis.

Mercury cell electrolysis, also known as the Castner-Kellner process, was the first method used to produce chlorine on an industrial scale. Titanium or graphiteanodes are located above a liquid mercury cathode. Slate baffles divide the cell into two chambers, in which the anode is in contact with just one. The baffles do not go all the way to the bottom of the cell, but allow the mercury cathode (but not the electrolyte) to flow beneath them. Sodium chloride solution is placed in the anode chamber and water in the other chamber. When an electrical current is applied, chlorine is released at the anodes and sodium dissolves into the mercury cathode forming an amalgam. By rocking the entire cell, the mercury amalgam is exposed to the water chamber, where it reacts to form sodium hydroxide and hydrogen gas as byproducts.[3][4]

This method consumes vast amounts of energy and there are also concerns about mercury emissions.

In diaphragm cell electrolysis, an asbestos (or other porous material) diaphragm separates cathode and anode, preventing the chlorine forming at the anode and the sodium hydroxide forming at the cathode from re-mixing. There are several variants of this process: the Le Sueur cell (1893), the Hargreaves-Bird cell (1901), the Gibbs cell (1908), and the Townsend cell (1904).[5][6] The cells vary in construction and placement of the diaphragm, with some having the diaphragm in direct contact with the cathode. Each of these uses the same principle of allowing sodium ions to diffuse through the porous diaphragm from the anode side containing the chloride to the cathode side containing the hydroxide. The concentration of sodium ions in the cathode side is kept low by constantly removing some hydroxide solution and replacing it with water. The sodium ion concentration on the anode side is kept high by adding sodium chloride to keep the solution saturated. Sodium ions are driven by the electric current to flow toward the cathode, whereas the chloride ions are driven in the opposite direction. Despite this some diffusion of chloride and hypochlorite ions through the diaphragm is unavoidable. As a result diaphragm methods produce alkali of less purity than do mercury cell methods. But diaphragm cells are not burdened with the cost of mercury and the problem of preventing mercury discharge into the environment. They also operate at a lower voltage, resulting in an energy savings over the mercury cell method.[6]

The electrolysis cell is divided into two by a cation permeable membrane acting as an ion exchanger. Saturated sodium chloride solution is passed through the anode compartment leaving a lower concentration. Sodium hydroxide solution is circulated through the cathode compartment exiting at a higher concentration. A portion of this concentrated sodium hydroxide solution is diverted as product while the remainder is diluted with deionized water and passed through the electrolyzer again.

This method is nearly as efficient as the diaphragm cell and produces very pure sodium hydroxide but requires very pure sodium chloride solution.

Before electrolytic methods were used for chlorine production, the direct oxidation of hydrogen chloride with oxygen or air was exercised in the Deacon process:

4 HCl + O2 → 2 Cl2 + 2 H2O

This reaction is accomplished with the use of CuCl2 as a catalyst and is performed in 400°C. The amount of extracted chlorine is approximately at 80%. Due to the extremely corrosive reaction mixture, industrial use of this method is difficult.

Another earlier process to produce chlorine was to heat brine with acid and manganese dioxide.

Small amounts of chlorine gas can also be made in the laboratory by putting concentrated hydrochloric acid in a flask with a side arm and rubber tubing attached. Manganese dioxide is then added and the flask stoppered. The reaction is not greatly exothermic. As chlorine is denser than air, it can be easily collected by placing the tube inside a flask where it will displace the air. Once full, the collecting flask can be stoppered.

Key to the production of chlorine is the operation of the brine saturation system. Maintaining a properly saturated solution with the correct purity is vital especially with membrane cells. Many plants have a salt pile which is sprayed with recycled brine. Others have slurry tanks that are fed raw salt. The raw brine is then treated with sodium hydroxide, sodium carbonate and a polymer flocculant to reduce calcium, magnesium and other impurities. The brine then proceeds to a large clarifier where the impurities are allowed to settle out. After overflowing the clarifier the brine is mechanically filtered via a sand bed filter and a leaf pressure filter before entering ion exchangers to further remove impurities. At several points in this process the brine is tested for hardness and strength. After the ion exchangers the brine is considered pure and is placed in large storage tanks to be pumped into the cell room. Brine fed to the cell line is heated to the correct temperature to control exit brine temperatures for a given electrical load. Brine exiting the cell room must be treated to remove residual chlorine, control pH, and be returned to the pile. This can be accomplished via dechlorination towers with acid and sodium bisulfite addition. Failure to remove chlorine can result in damage to the cells. Brine should be monitored for accumulation of chlorate and sulfate and either have treatment systems in place or purging of the brine loop to maintain safe levels since these too can damage cells.

A large building to house the many electrolytic cells is usually called a cell room or cell house. It contains support structures for the cells and connections for supplying electrical power to the cells. Also in this building is piping for supplying feed brine, feed caustic and the removal of spent brine, exit caustic as well as wet chlorine gas and wet hydrogen gas. Monitoring and control of the temperatures of the feed caustic and brine is done to control exit temperatures. The exit temperatures are usually monitored on a per cell basis and controlled via adjusting the feed temperatures and individual flow rates. Also monitored are the voltages of each cell which vary with the electrical load on the cell room which is used to control the rate of production. Monitoring and control of the pressures in the chlorine and hydrogen headers is also performed via pressure control valves.

Direct electrical current is supplied via a rectifier with the cells connected in series. Plant load is controlled by varying the current to the cell room based on the ratio of amperage to square area of membrane surface in the cell room. As the current is increased flow rates for brine, caustic and deionized water are increased while lowering the feed temperatures.

Chlorine gas exiting the cell line must be cooled and dried since the exit gas can be over 80º C and contains moisture that allows chlorine gas to be corrosive to iron piping. Cooling the gas allows for a large amount of moisture to condense and fall out of the gas stream where it can be piped off and placed in the exit brine to recover some of the chlorine and conserve deionized water. Cooling also improves the efficiency of the compression and liquefaction stage that follows. This usually accomplished via a multistage tube and shell set of coolers fed with cooling water on the first stage and chilled water on the second. Chlorine exiting the second stage is ideally between 18º C and 25º C. After cooling the gas stream passes through a series of packed towers with counter flowing sulfuric acid. These towers progressively remove any remaining moisture from the chlorine gas. After exiting the drying towers a filter system to remove any sulfuric acid droplets from the flow is usually employed.

Several methods of compression my be used. For small production levels a liquid ring compressor may be utilized. Medium sized facilities may select a reciprocating compressor. Large facilities typically employ a centrifugal compressor. The chlorine gas is compressed at this stage and may be further cooled by inter and after coolers. After compression it flows to the liquefiers, which are large freon chillers, where it is cooled enough to liquefy. Non condensible gases and trace chlorine gas are vented off as part of the pressure control of the liquefaction systems. This pressure control helps maintain the correct pressure for liquefaction to occur. These gases are routed to a gas scrubber, producing sodium hypochlorite, or utilized in the production of hydrochloric acid via an hydrochloric acid burner or oven.

Caustic fed to the cell room is flowing in a loop that is simultaneously being bled off to storage with a part diluted with deionized water and returned to the cell line for strengthening with in the cells. The caustic exiting the cell line must be monitored for strength in order to maintain the correct concentrations that allow for safe operation. Too strong or too weak of a solution may damage the cells. Membrane cells typically produce caustic in the range of 30% to 33% by weight. The caustic is heated at low electrical loads to control the exit temperature of the caustic. At higher loads it becomes necessary to cool the caustic to maintain correct exit temperatures. A tube and shell heat exchanger is typically employed in a dual mode configuration. The caustic exiting to storage is pulled from a storage tank and may be diluted to make a dilute 25% solution for sale to customers who require weak caustic or for use on site. Another stream may be pumped into a multiple effect evaporator set to produce 50% caustic, improving storage and allowing for economical transportation. The 50% and 25% caustic are stored in separate tanks and rail cars and tanker trucks are loaded at loading stations via pumps.

Hydrogen produced may be vented unprocessed directly to the atmosphere or cooled, compressed and dried for use in other processes on site or sold to a customer via pipeline. Some possible uses are hydrochloric acid or hydrogen peroxide production and use as a fuel in boilers or fuel cells.

Chlorine is also used widely in the manufacture of many every-day items, or to purify water in various forms.

Used (in the form of hypochlorous acid) to kill bacteria and other microbes in drinking water supplies and swimming pools. However, in most non-commercial swimming pools chlorine itself is not used, but rather sodium hypochlorite (household bleach), a compound of chlorine with sodium and oxygen. Calcium hypochlorite is also used as a cheaper alternative. Even small water supplies are now routinely chlorinated.[8] (See alsochlorination)

Chlorine gas is an oxidizer; it undergoes halogen substitution reactions with lower halide salts. For example, chlorine gas bubbled through a solution of bromide or iodide anions oxidizes them to bromine and iodine respectively.

Like the other halogens, chlorine participates in free-radical substitution reactions with organic compounds. This reaction usually results in a mixture of products based on a statistical distribution. It is often difficult to control the degree of substitution as well — multiple substitutions are common. Unless the different reaction products are easily separated, e.g. by distillation, free-radical chlorination is not a preferred synthetic route.

Like the other halides, chlorine undergoes electrophilic additions reactions, most notably, the halogenation of alkenes, and the halogenation of aromatic compounds with a Lewis acid catalyst. Organic chloride compounds tend to be less reactive than the corresponding bromides or iodides, but they tend to be cheaper. They may be activated for reaction by substituting with a tosylate group, or by the use of a catalytic amount of sodium iodide.

Chlorine gas has been used by insurgents in the Iraq War as a chemical weapon to increase the capability to threaten the local population and coalition forces in their attacks. On March 17, 2007, for example, three chlorine filled trucks were detonated in the Anbar province killing 2 and sickening over 350.[9] Other chlorine bomb attacks resulted in higher death tolls, with more than 30 deaths on two separate occasions.[10] While fatalities are common, chlorine bombings do not inflict a massive loss of life, and are primarily intended to create widespread panic.

Chlorine is a toxic gas that irritates the respiratory system. Because it is heavier than air, it tends to accumulate at the bottom of poorly ventilated spaces. Chlorine gas is a strong oxidizer, which may react with flammable materials. For more information see an MSDS.